For general towing, I keep the rpms up, and don't have the pedal to the floor, going up long hills. Slow and steady. Should keep your coolant cooler. I know you don't want to be that slow guy going up hills, but you don't want to be that guy with the hood up on the side of the road either. As mentioned, 5000lbs should've a cake walk for your V10.
For an auto, if you can keep the torque converter locked up, that will keep your tranny temps lower. Diesel trucks have a way you could wire up a switch to keep it locked up. I don't have the switch set up in my 01 diesel, but when the torque converter is locked, the transmission temp stays below 160F. That's pulling 10,000lbs. With some research, I'm sure you could wire in a switch to do the same for the V10.